Latest revision as of 01:52, 16 January 2009

The following content is a copy of the /usr/src/linux/Documentation/CodingStyle file (modified slightly for visual presentation). A current copy can also be found online here.

Linux kernel coding style

This is a short document describing the preferred coding style for the
linux kernel. Coding style is very personal, and I won't _force_ my
views on anybody, but this is what goes for anything that I have to be
able to maintain, and I'd prefer it for most other things too. Please
at least consider the points made here.

First off, I'd suggest printing out a copy of the GNU coding standards,
and NOT read it. Burn them, it's a great symbolic gesture.

Chapter 1: Indentation

Tabs are 8 characters, and thus indentations are also 8 characters.
There are heretic movements that try to make indentations 4 (or even 2!)
characters deep, and that is akin to trying to define the value of PI to
be 3.

Rationale: The whole idea behind indentation is to clearly define where
a block of control starts and ends. Especially when you've been looking
at your screen for 20 straight hours, you'll find it a lot easier to see
how the indentation works if you have large indentations.

Now, some people will claim that having 8-character indentations makes
the code move too far to the right, and makes it hard to read on a
80-character terminal screen. The answer to that is that if you need
more than 3 levels of indentation, you're screwed anyway, and should fix
your program.

In short, 8-char indents make things easier to read, and have the added
benefit of warning you when you're nesting your functions too deep.
Heed that warning.

Don't put multiple statements on a single line unless you have
something to hide:

if (condition) do_this;
do_something_everytime;

Outside of comments, documentation and except in Kconfig, spaces are never
used for indentation, and the above example is deliberately broken.

Get a decent editor and don't leave whitespace at the end of lines.

Chapter 2: Breaking long lines and strings

Coding style is all about readability and maintainability using commonly
available tools.

The limit on the length of lines is 80 columns and this is a hard limit.

Statements longer than 80 columns will be broken into sensible chunks.
Descendants are always substantially shorter than the parent and are placed
substantially to the right. The same applies to function headers with a long
argument list. Long strings are as well broken into shorter strings.

Chapter 3: Placing Braces

The other issue that always comes up in C styling is the placement of
braces. Unlike the indent size, there are few technical reasons to
choose one placement strategy over the other, but the preferred way, as
shown to us by the prophets Kernighan and Ritchie, is to put the opening
brace last on the line, and put the closing brace first, thusly:

if (x is true) {
we do y
}

However, there is one special case, namely functions: they have the
opening brace at the beginning of the next line, thus:

int function(int x)
{
body of function
}

Heretic people all over the world have claimed that this inconsistency
is ... well ... inconsistent, but all right-thinking people know that
(a) K&R are _right_ and (b) K&R are right. Besides, functions are
special anyway (you can't nest them in C).

Note that the closing brace is empty on a line of its own, _except_ in
the cases where it is followed by a continuation of the same statement,
ie a "while" in a do-statement or an "else" in an if-statement, like
this:

do {
body of do-loop
} while (condition);

and

if (x == y) {
..
} else if (x > y) {
...
} else {
....
}

Rationale: K&R.

Also, note that this brace-placement also minimizes the number of empty
(or almost empty) lines, without any loss of readability. Thus, as the
supply of new-lines on your screen is not a renewable resource (think
25-line terminal screens here), you have more empty lines to put
comments on.

Chapter 4: Naming

C is a Spartan language, and so should your naming be. Unlike Modula-2
and Pascal programmers, C programmers do not use cute names like
ThisVariableIsATemporaryCounter. A C programmer would call that
variable "tmp", which is much easier to write, and not the least more
difficult to understand.

HOWEVER, while mixed-case names are frowned upon, descriptive names for
global variables are a must. To call a global function "foo" is a
shooting offense.

GLOBAL variables (to be used only if you _really_ need them) need to
have descriptive names, as do global functions. If you have a function
that counts the number of active users, you should call that
"count_active_users()" or similar, you should _not_ call it "cntusr()".

Encoding the type of a function into the name (so-called Hungarian
notation) is brain damaged - the compiler knows the types anyway and can
check those, and it only confuses the programmer. No wonder MicroSoft
makes buggy programs.

LOCAL variable names should be short, and to the point. If you have
some random integer loop counter, it should probably be called "i".
Calling it "loop_counter" is non-productive, if there is no chance of it
being mis-understood. Similarly, "tmp" can be just about any type of
variable that is used to hold a temporary value.

If you are afraid to mix up your local variable names, you have another
problem, which is called the function-growth-hormone-imbalance syndrome.
See next chapter.

Chapter 5: Functions

Functions should be short and sweet, and do just one thing. They should
fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
as we all know), and do one thing and do that well.

The maximum length of a function is inversely proportional to the
complexity and indentation level of that function. So, if you have a
conceptually simple function that is just one long (but simple)
case-statement, where you have to do lots of small things for a lot of
different cases, it's OK to have a longer function.

However, if you have a complex function, and you suspect that a
less-than-gifted first-year high-school student might not even
understand what the function is all about, you should adhere to the
maximum limits all the more closely. Use helper functions with
descriptive names (you can ask the compiler to in-line them if you think
it's performance-critical, and it will probably do a better job of it
than you would have done).

Another measure of the function is the number of local variables. They
shouldn't exceed 5-10, or you're doing something wrong. Re-think the
function, and split it into smaller pieces. A human brain can
generally easily keep track of about 7 different things, anything more
and it gets confused. You know you're brilliant, but maybe you'd like
to understand what you did 2 weeks from now.

Chapter 6: Centralized exiting of functions

Albeit deprecated by some people, the equivalent of the goto statement is
used frequently by compilers in form of the unconditional jump instruction.

The goto statement comes in handy when a function exits from multiple
locations and some common work such as cleanup has to be done.

The rationale is:

- unconditional statements are easier to understand and follow
- nesting is reduced
- errors by not updating individual exit points when making modifications are prevented
- saves the compiler work to optimize redundant code away ;)

Chapter 7: Commenting

Comments are good, but there is also a danger of over-commenting. NEVER
try to explain HOW your code works in a comment: it's much better to
write the code so that the _working_ is obvious, and it's a waste of
time to explain badly written code.

Generally, you want your comments to tell WHAT your code does, not HOW.
Also, try to avoid putting comments inside a function body: if the
function is so complex that you need to separately comment parts of it,
you should probably go back to chapter 5 for a while. You can make
small comments to note or warn about something particularly clever (or
ugly), but try to avoid excess. Instead, put the comments at the head
of the function, telling people what it does, and possibly WHY it does
it.

Chapter 8: You've made a mess of it

That's OK, we all do. You've probably been told by your long-time Unix
user helper that "GNU emacs" automatically formats the C sources for
you, and you've noticed that yes, it does do that, but the defaults it
uses are less than desirable (in fact, they are worse than random
typing - an infinite number of monkeys typing into GNU emacs would never
make a good program).

So, you can either get rid of GNU emacs, or change it to use saner
values. To do the latter, you can stick the following in your .emacs file:

This will define the M-x linux-c-mode command. When hacking on a
module, if you put the string -*- linux-c -*- somewhere on the first
two lines, this mode will be automatically invoked. Also, you may want
to add

to your .emacs file if you want to have linux-c-mode switched on
automagically when you edit source files under /usr/src/linux.

But even if you fail in getting emacs to do sane formatting, not
everything is lost: use "indent".

Now, again, GNU indent has the same brain-dead settings that GNU emacs
has, which is why you need to give it a few command line options.
However, that's not too bad, because even the makers of GNU indent
recognize the authority of K&R (the GNU people aren't evil, they are
just severely misguided in this matter), so you just give indent the
options "-kr -i8" (stands for "K&R, 8 character indents"), or use
"scripts/Lindent", which indents in the latest style.

"indent" has a lot of options, and especially when it comes to comment
re-formatting you may want to take a look at the man page. But
remember: "indent" is not a fix for bad programming.

Chapter 9: Configuration-files

For configuration options (arch/xxx/Kconfig, and all the Kconfig files),
somewhat different indentation is used.

Generally, CONFIG_EXPERIMENTAL should surround all options not considered
stable. All options that are known to trash data (experimental write-
support for file-systems, for instance) should be denoted (DANGEROUS), other
experimental options should be denoted (EXPERIMENTAL).

Chapter 10: Data structures

Data structures that have visibility outside the single-threaded
environment they are created and destroyed in should always have
reference counts. In the kernel, garbage collection doesn't exist (and
outside the kernel garbage collection is slow and inefficient), which
means that you absolutely _have_ to reference count all your uses.

Reference counting means that you can avoid locking, and allows multiple
users to have access to the data structure in parallel - and not having
to worry about the structure suddenly going away from under them just
because they slept or did something else for a while.

Note that locking is _not_ a replacement for reference counting.
Locking is used to keep data structures coherent, while reference
counting is a memory management technique. Usually both are needed, and
they are not to be confused with each other.

Many data structures can indeed have two levels of reference counting,
when there are users of different "classes". The subclass count counts
the number of subclass users, and decrements the global count just once
when the subclass count goes to zero.

Examples of this kind of "multi-level-reference-counting" can be found in
memory management ("struct mm_struct": mm_users and mm_count), and in
filesystem code ("struct super_block": s_count and s_active).

Remember: if another thread can find your data structure, and you don't
have a reference count on it, you almost certainly have a bug.

Chapter 11: Macros, Enums, Inline functions and RTL

Names of macros defining constants and labels in enums are capitalized.

#define CONSTANT 0x12345

Enums are preferred when defining several related constants.

CAPITALIZED macro names are appreciated but macros resembling functions
may be named in lower case.